A Ti alloy is of a light weight and high strength, as well as being excellent in corrosion resistance, as compared with a metallic material such as iron or its alloy. Therefore, the Ti alloy is broadly used as a pipe material, a plate material or a wire material, or as a Ti alloy member produced by secondary working using these materials, for sport and leisure equipments, medical implements, various plant parts and members, and aerospace equipments.
In the meantime, it is known that a metallic material is corroded in an acid solution, and causes hydrogen-embrittlement phenomena by absorbing hydrogen when hydrogen exists therearound at a hydrogen absorbing potential. A Ti alloy, as well as steal, aluminium alloy, copper alloy and nickel alloy are known as metals that are easy to cause hydrogen-embrittlement.
The hydrogen-embrittlement may not only shorten the life of the Ti alloy member in the above field of use of the Ti alloy, but also cause a phenomenon, such as stress corrosion cracking.
In order to address the above, there are hitherto known a method that inhibits hydrogen from being absorbed into the inside of a Ti alloy member by the condition of the forming process, such as surface treatment, of the Ti alloy member, and a method that inhibits the absorption of hydrogen by the composition of a Ti alloy.
As the former method, Patent Document 1 describes that a surface of a Ti alloy member is polished and then an oxide film is formed thereon. Patent Document 2 describes that when forming a Ti alloy member, titanium carbide, titanium nitride or titanium carbonitride is prevented from being formed on a surface thereof. Patent Document 3 describes that a Ti alloy member is formed so as to form an acicular a structure having a given aspect ratio.
As the latter method, Patent Document 4 and Patent Document 5 describe that absorption of hydrogen of a Ti alloy is inhibited by containing a given amount of Al in Ti.
However, none of these methods can produce a satisfactory effect to inhibit a Ti alloy from absorbing hydrogen, and therefore a conventional Ti alloy may cause hydrogen-embrittlement by absorbing hydrogen.
Patent Document 1: Japanese Patent Application Laid-open No. Sho-53-12737
Patent Document 2: Japanese Patent Application Laid-open No. Hei-6-2175
Patent Document 3: Japanese Patent Application Laid-open No. Hei-10-88258
Patent Document 4: Japanese Patent Application Laid-open No. 2003-129152
Patent Document 5: Japanese Patent Application Laid-open No. 2005-36314